1. Field of the Invention
The present invention relates to an apparatus and to a method for determining the shape of an end of a welding bead (welding part) formed by irradiating a welding material with a welding laser. More specifically, the present invention relates to an algorithm to, in laser lap welding, capture an image of an end of a welding bead with a high-speed camera and to determine whether there is a hole defect at the end of the welding bead based on the captured image in a very short period of time immediately after the welding and before thermoluminescence from the metal ends disappears.
2. Description of Related Art
There has heretofore been proposed a method for detecting a weld defect in welding such as laser welding by obtaining a signal from a bead region (welding region) by a camera, an optical sensor and the like and analyzing the detected signal (e.g., JP 2007-326134 A (hereinafter referred to as Patent Document 1) and JP 2005-230913 A (hereinafter referred to as Patent Document 2)).
In Patent Document 1, a signal from a weld spot is taken by use of a thermal radiation sensor and a reflection sensor in a welding process, and then whether or not non-defective welding is possible is determined by comparing the change of the detected signal over time with a table of reference data associated with welding conditions. Thus, the method disclosed in Patent Document 1 prevents weld cracks or weld defects after the welding.
In addition, in Patent Document 2, a weld region is irradiated with a measuring beam, and a temperature signal obtained immediately after the welding is compared to experimentally obtained upper and lower temperature limits. Then, in Patent Document 2, a weld defect is determined by counting the number of times that the temperature signal exceeds the upper and lower temperature limits.
FIG. 11 shows an example of a case in which a signal is continuously detected from the bead region using a camera, an optical sensor and the like in laser welding.
The determination method using the detected signal as in Patent Document 1 is known to involve a phenomenon in which the detected signal at the end of the bead greatly varies when welding laser radiation and bead region monitoring are stopped at the end of the bead region, as shown in FIG. 11. For this reason, there is a problem that a weld defect at the end of the bead region cannot be determined with the same determination method as that used during the welding process.
In addition, the method disclosed in Patent Document 2 determines the weld defect based on how many times the temperature signal obtained immediately after the welding exceeds the limit values. In this method, the shape itself of the end of the bead region is not observed at all. For this reason, a weld defect at the end of the bead region may not be accurately determined. Moreover, the method disclosed in Patent Document 2 also requires an apparatus for emitting a measuring laser in addition to the welding laser, leading to a problem of increasing the cost of the apparatus.
FIGS. 12A and 12B show images of the end of the bead region captured with a camera. FIG. 12A shows the image with a hole defect at the end of the bead region, whereas FIG. 12B shows the image in a case in which the welding is normally completed. As shown in FIG. 12A, when a hole defect occurs at the end of the bead region, the end of the bead region is made concave in the extending direction of the bead. On the other hand, as shown in FIG. 12B, when the welding is normally completed, the end of the bead region is made convex in the extending direction of the bead.
Meanwhile, there has heretofore been proposed a method for examining a hole defect at the end of the bead region after a welding process is fully completed. For example, in what is called a backlight type examination method, illumination light is emitted onto a weld face from the back in a separate step after the laser welding process is fully completed, and the presence of a hole defect is determined by detecting the light leaking to the front. However, this method has a problem of reduction in production efficiency since the examination process is added as an extra step after the welding is completed.